Magnetic core structure



Inf s `Patentod Nov. 15, 1949 UNITED STATES PATENT OFFICE MAGNETIC CORE STRUC'I'UBE James G. Ford, Sharon, and Clifford C. Horstman and John H. Bramble, Sharpsville, Pa., alimors to Westlnghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 9, 1946, Serial No. 882,184

6 Claims. (Cl. 175-356) This invention relates to electrical apparatus such as transformers, and particularly to magnetic core structures for use in such electrical apparatus.

Magnetic steel has been developed in recent years having preferably good magnetic characteristics in the direction of rolling the steel during the process of its manufacture which are vbetter than in other directions, and which are better than the magnetic properties of commercial grades of silicon steel formerly employed. Such improved magnetic steel is characterized by low watts loss and high permeability in the direction of the grain Orientation of the steel, that is, in the direction in which the magnetic material is rolled during its process of manufacture. In order to take advantage of the better magnetic properties of such newer magnetic steel, it is necessary to so use the steel that the magnetic fiux passes through it in the direction of rolling. When so used the fiux density may be much higher than formerly employed. With the advent of more efllcient magnetic steels which permit the core structure to be operated at higher magnetic induction than was formerly the practice, the amount of steel required to form a core structure of a transformer having a given power rating has been greatly reduced.

In order to take advantage of this characteristic of the newer high grade magnetic steels, a form of magnetic circuit structure has been developed comprising one or more core loops made of magnetic sheet steel ribbon wound layer upon layer on a shaping form, such as a steel mandrel, having the desired dimensions. The core loops so formed are usually provided with substantially rectangular windows or openings therethrough. The mandrels are designed to correspond substantially in size and shape to the desired size and shape of the window in the finished core loop, the window in the core loop being bound by the fiat side of the inner turn of the ribbon of steel forming the core loop. The core loops so formed, in most cases remaining on the mandrels on which they are wound, are then annealed to relieve strains in the sheet steel ribbon caused by the winding operation and fully develop its magnetic quality. The loops are placed on their sides in rows and are then loaded by applying weights so as to preserve the form of the rectanguler core loop during the annealing process. The wound and annealed core loops are then vacuum impregnated with a plastic adhesive bonding material which flows in the space between the successive turns or layers of magi netic sheet steel, after which the core loop structures are baked so that the bonding material adheres to the layers of sheet steel, forming a solid structure. The bonding material may be a thermoplastic material such as a vinyl acetate 2 resin or vinyl acetal resin with slight additions of phenolic type or hydrolyzed vinyl acetate resins.

These core loop structures are further processed by cutting the loop to form two U-shaped parts adapted to be reassembled about the circuit conducting coils. The cut surfaces of the core may then be ground to insure smooth machined butt joint surfaces and treated with an etchant to remove any short circuiting burr produced by the cutting or grinding action.

A problem has arisen in connection with the use of this type of core in transformers and especially in the larger sizes. It is a well known fact that a butt joint in a magnetic circuit excited from 60 cycle alternating current will cause av large 120 cycle Vibration with a resultant high noise level if the two core parts comprising the butt joints are not clamped tightly together. In the simple case it is necessary to only apply enough pressure on the butt joint to overcome the magnetic force of attraction between the core pieces so as to always have a positive force holding together the two pieces which comprise the butt joint. This is true, providing the core legs that are brought together to form the butt joint are sufllciently rigid to preclude anyV transverse movement or the collapsing of the core leg due to the movement of the individual laminations.

Experience has shown that the use of a thermoplastic bond for the impregnation of the core loop structure, such as the vinyl acetate resins or Vinyl acetal resins mentioned above, with slight additions of phenolic types or hydrolized vinyl acetate type resins as employed with this type of bonding material is very satisfactory with the smaller sized cores. However, this type of bonding material is not sufilciently rigid at operating temperatures in the larger core structures under the bonding forces readily available to hold. the butt joint surfaces together.

It is an object of the invention to provide a core of the general characteristics above described so modified from the conventional strucf ture as to eliminate or greatly reduce the noise level of the core to an insignificant value.

It is a further object of the invention to provide a core of the above indicated character that is stable in the transformer oil up to 100 C. temperature, that is, in which the transformer core legs uniting to form the butt joints will not move so as to disturb the full surface contact of the abutting face at the butt joints.

Other objects and advantages will be apparent from the following description of a preferred embodiment thereof, reference being had to the ac-. companying drawings in which:

Fig. l is an elevational view of a conventional core and coil assembly,

Fig. 2 is a plan view of the assembly shown in Fig. l.

Fig. 3 is a perspective view of the magnetic material in a core loop of the type shown in Figs. i and 2.

Fig. 4 is an elevational view of such a core loop modified to disclose one type of distortion to which the loop is subject.

Fig. 5 is an elevational view of such a core loop modified to disclose another type of distortion to which the core loop is subject.

Fig. 6 is an elevational view of a core loop constructed in accordance with the invention.

Fig. 7 is an elevational view of the conventional core loop shown in Fig. 3, diagrammatlcally illustratlng the effect of certain forces, and

Fig. 8 is an elevational view of a 3 phase core structure in which the principles of the invention are applied.

Referring to the drawing and particularly to Figs. 1 and 2 thereof, a conventional core and coil assembly of the transformer is illustrated including a coil structure i, which may comprise a plurality of primary and secondary windings arranged in a well known manner in the winding space outlined in the drawing, and about which two core loops or magnetic circuits 2 and 3 are positioned, each loop comprising an upper section l and a lower section I. The core loops are formed by winding a continuous strip of magnetic sheet steel ribbon, which material is then annealed, vacuum impregnated with bonding material and cut along the lines shown at 6 and 1 as above described to form the upper and lower U-shaped core sections 4 and 5 that may be suitably assembled about the coil structure. After the faces of the upper and lower core sections I and 5 have been worked smooth and etched in the manner above described, a suitable resin is applied to the faces which met to form the butt ioints at 6 and 'l when the sections are assembled about the coil structure. A banding or clamping strap ls applied to each of the core loops extending thereabout and stretched by means of a tool which pulls the opposite ends of the strap through a coupling sleeve 9. While the band or strap 0 is held under a predetermined tension, the coupling sleeve 9 is crimped to force portions of the outer edges of the sleeve 9, together with the associated portions of the outer edges of the ends of the banding strap 8 to form a firm coupling. The structure is then placed in an oven for drying it, and as it is heated, the resin applied between the surfaces of the butt Joint fiows out to a limited extent, thus slightly decreasing the pressure between the faces of the joint. The band or strap l is stretched a suilicient amount when applied and has suflicient elasticity to maintain the desired pressure on the .ioints B and 1, even though the resin material flows during cure. Btresses of the order of 25,000 pounds per square inch are developed in the banding strap 8. It has been found that stresses of this order in the banding strap will satisfactorily take care of the change caused by the flow of the resin material in the .ioints 0 and 1 and still maintain satisfactory pressure. Films of the resin material of the order of .00025 inch in thickness have been found to provide a satisfactory insulating film and protect the worked edges of the core sections I and from any acid or moisture in the dielectric liquid in which the core and coll assembly operates when assembled in its casing.

Fig. 3 is a perspective view of the active material in a loop of the character described. comprising the upper and lower core sections 4 and l. In order to maintain full surface contact of the abutting faces of the core sections l and I at the butt Joints 0 and 1, upon application of pressure thereto it is necessary that the core legs l l and I! are sufliciently rigid to preclude any lateral movement thereof or collapsing.

It is necessary for the core to be absolutely stable under all conditions of operation in the transformer oil up to C. temperature. If any movement of the legs takes place, it will change the full surface contact at the butt Joint and experience has indicated that the core will have an obiectionably high sound level. A type of distorticn that may be realized with these types of thermoplastic bonds is lllustrated in Fig. 4. In the figure, the degree of distortion is somewhat exaggerated so that it can be more readily observed. This type of distortion is characterized in that the deformity causes the inner edges of the core loop parts adjacent the window as shown at ll and |5 to be in contact while the surfaces forming the butt joints at I and 'I are separated so that they are spaced apart at IS and H, leaving a slight opening between the contact surfaces that should unite so that the abutting faces on the opposite sides of the joints are in 'full contact over their entire surfaces.

Recently new types of core bonding material have been developed having thermoset characteristics. The result is a more suitable core structure than one employing the thermoplasticl material, resultinl in a core that will not collapse in the manner shown in Fig. 4 under operatng conditions up to 100 C. in oil. The thermoset bonding material vmay be a phenolformaldehyde resin or melamine formaldehyde resin. However, the use of this bonding material does not of itself solve the problem because, even with the thermoset bonding material the core structure may be unstable. We have found that the following sequence of events takes place when the core is subjected to a temperature cycle from 25 C. and 100 C., such as may bc experienced in the ordinary operation of the transformer. The core surfaces that are brought into engagement to form the butt joint are ground accurately and the core as assembled will be o ulet. It will remain quiet when heated to 100 C., but subsequent cooling will become noisy and the sound level will be high enough to be commercially objectionable. Analysis shows that on heating and subsequent cooling the core parts assume a shape as illustrated in Fig. 5. As shown in Fig. 5. the upper edges of the core loop at IB and I9 are in engagement and the surfaces of the core parts 4 and 5 adjacent the butt ioints are spaced apart or diverge toward the opening of the window at 22 and 23, so that the surfaces on opposite sides of the butt Joint do not smoothly engage to make an efficient butt joint and the ends of the laminations are therefore relatively free to vibrate under the influence of the reversing magnetic flux and to cause a loud noise which would not occur if the adjoinlng surface forming the butt Joint fitted smoothly throughout their entire adioining faces.

It is believed that with the use of the harder thermoset bonding material, the core legs 21 and 25 are rigid but that, during the heating and cooling sequence. a diil'erential contraction of the bonding material and steel takes place at the corners of the yoke, that is in the areas identified by the numerals 20, 21, 20 and 29. This causes the winding legs 24 and 25 to pull inwardly to anaan somewhat narrow the window in the area adjacent the butt joints, thus causing the opposite faces of the joints to diverge slightly. This divergence prevents the opposite faces, forming the butt joints to meet under even pressure and thus naturally results in a noisy core. The general tendency of this type of core is to be quiet when hot and to become noisy on cooling. The magnitude of these movements in very small. The gap between the surfaces at 22 and 23 may perhaps be only .0005 to .001 inch, but this is suftlcient to increase the sound level from 34 decibels to 60-70 decibels for a core welghing about 60 pounds. This condition is recurrent and will continue on successive heating sequences.l

Thus the core structure is not stable and therefore unsatisfactory.

Fig. 6 illustrates a core loop constructed in I accordance with the invention and differs from the conventional core loop shown in Figs. l, 2 and 3 in that the central window 3| is formed with' a central bulge, that is, the inner surfaces 32 and likewise the outer surfaces 33 of the winding legs of the core are curved away from each other. so that the leg portions 34 are arch shaped. When, therefore, the two U-shaped portions 38 and 31 each comprising two leg portions 34 joined by a yoke portion 38 are brought into engagement along the butt joint surfaces 4| and 42, and the two U-shaped portions forming the core loop are forced together by a steel band 43 and a coupling sleeve 46 in a manner similar to the band 8 and the coupling sleeve 9 of Figs. 1 to 5, the force applied to the butt joints will not be directly in opposition to each other.

The core loop is formed during the winding operation so as to have a slght radius` as indicated by the inner and outer surfaces 32 and 33, respectively. This bulge in the central portion ofthe window is indicated by the offset X. With a thermoset bond, this core will be suitable under all operating conditions when banded, whereas the conventional core having straight legs will not be stable for the reasons partly explained before and partly given in comprising Figs. 6 and '7.

Fig. '7 diagrammatically illustrates the direction of the forces applied to the butt joints shown in Figs. 1 to 5. When the band 8 is applied under tension, this tension results in equal and diametrically opposite forces F and F' applied to opposite sides of the surfaces that meet to form the butt joints 6 and l. If the movement such'as described above occurs in the corners or yolre portions of the core during the heat cycle we have in effect, a toggle joint, which is unstable and can move in any direction as shown by IJU the double arrows M. In practice this results in 1 an opening in the butt joint, or divergence in the butt joint surfaces as shown by the dotted lines in one leg. Since the forces are directly in line with the direction of the winding leg, there is no area of contact of the joint to resist this movement. The net result is a noisy core.

This unstable condition discussed with respect to Fig. 'f is removed by curving the winding legs outwardly as shown in Fig. 6, since the oIJPOSing forces E and E' along the winding leg portions of the sections and 31 are not diametrically op-` posite and a small resultant force E is present. This resultant force acts outwardly and opposes any force which would tend to cause the legs to move inwardly, thus preventing any inward movement of the winding legs at the butt joints 4l and 42. Outward moveinent of the legs is prevented by the steel band 43 surrounding the two core sections 36 and 31. The net result is a stable core structure under heat cycling conditlons because there is no unstable joint effect present. The core joint surfaces remain in good contact over their entire surfaces under all conditions of operation and the core is quiet. For best results, the butt joints should be in the center of the winding leg midway between the yoke portions 38. It has been found that a practical dimension for X is about 2% of the length of the Winding leg.

Fig. 8 illustrates a 3-phase core employing the above described features of the invention. In the core shown in Fig. 8, three separate core loops 5l, 52 and 53 are provided, the two smaller loops being positioned within the larger loop 53. The

two inner loops 5| and 52 are formed corresponding to the loops shown in Fig. 6 illustrating the invention and the outer loop 53 follows the outer curvature of the loops 5| and 52. A steel band 541 is applied under tension about the three loops and the two ends thereof are held together by the coupling sleeve 55, for maintaining a substantially constant pressure on the adjoining surfaces comprising the butt joints 55. The opposing forces that are applied by the steel band 54 to the adjoining surfaces that engage to form the butt joints 56 are in directions corresponding to the arrows E and E' in Fig. 6, so that there is a horizontal component of force at the butt joints in directions away from the window 3l. Thus a stable core structure results.

Modifications may be made in the specific structure illustrated and described without departing from the spirit of the invention, but we do not wish to be limited otherwise than in' accordance with the scope of the accompanying claims.

We claim as our invention:

1. In an electrical apparatus, a magnetic core structure comprising a core loop formed from a ribbon of magnetic sheet steel wound flatwise layer upon layer about a substantially rectangular window presenting a side surface of the ribbon steel along the surface of the core outlining the window and having two opposite sides longer than the remaining sides, the longer sides of the core loop being formed with a slight curvature outwardly from each other, the departure of each longer side of the core loop from a straight line between its ends being in the order of two per cent of the length of the side with the greatest departure of the side from a straight line being at the miclpoint of the side, the core loop comprising two separate U-shaped pieces having abutting joints midway between the respective ends of the two longer sides of the core loop formed by the two pieces, the layers of magnetic sheet steel ribbon comprising each of said U- shaped pieces being solidly held together With intervening thin layers of a bending material adhering to the layers of magnetic sheet steel ribbon, and a resilient strap of steel applied under tension about the two U-shaped pieces to maintain constant pressure between their adjoining butt joint surfaces.

2. In an electrical apparatus, a magnetic core structure comprising a core loop formed from a ribbon of magnetic sheet steel wound flatwise layer upon layer about a substantially rectangular window presenting a side surface of the ribbon steel along the surface of the core outlining the window and having two opposite sides longer than the remaining sides, the longer sides of the core loop being formed with a slight curvature outwardly from each other the` departure of each longer side of the core loop from a straight line between its ends being in the order of two per cent of the length of the side with the greatest departure of the side from a straight line being at the midpoint of the side, the two longer sides being cut midway between their ends to provide two separate U-shaped core pieces having adjoining butt Joint surfaces at their ends which are in engagement along the line of the cut to provide low-loss joints, the layers of magnetic sheet steel ribbon comprising each of said U-shaped pieces being solidly held together with intervening thin layers of a bonding material adhering to the layers of magnetic sheet steel ribbon, and a resilient strap of steel applied under tension about the two U-shaped pieces to maintain constant pressure between their adjoining butt joint surfaces.

3. In an electrical apparatus, a magnetic core structure comprising a core loop formed from a ribbon of magnetic sheet steel wound fiatwise layer upon layer about an eiongated window and having two opposite sides longer than the remaining two sides, the two longer sides of the core loop being formed with a slight bow outwardfrom each other along the central portions thereof, the departure of each longer side of the core loop from a straight line between its ends V being in the order of two per cent of the length of the side with the greatest departure of the side from a straight line being at the midpoint of the side, said two longer sides being cut across intermediate their ends forming two substantia'ily U-shaped core loop sections having adjoining butt Joint surfaces midway between the ends of said two longer sides. the adioining surfaces, being machine worked and etched to form close fitting smooth butt joint surfaces, the layers of magnetic sheet steel ribbon comprising each of said U-shaped sections being solidly held together with intervening thin layers of a bonding material adhering to the layers of magnetic sheet steel ribbon, and a resilient band of metal applied under tension about the two U-shaped core sections to maintain constant pressure between their adjoining butt joint surfaces.

4. In an electrical apparatus, a magnetic core structure comprising a core loop formed from a ribbon of magnetic sheet steel wound flatwise layer upon layer about a substantially rectangular window presenting a side surface of the ribbon steel along the surface of the core outlining the window and having two opposite sides longer than the remaining sides. the longer sides of the core loop being formed with a slight curvature outwardly from each other, the departure of each longer side of the core loop from a straight line between its ends being in the order of two per cent of the length of the side with the greatest departure of the side from a straight line being at the midpoint' of the side, the core loop comprising two similar U-shaped pieces having leg portions of equal length terminating at their ends in smooth flat surfaces, the flat surfaces of the U-shaped pieces being in engagement to form butt Joints in the longer sides of the core loop, the layers of magnetic sheet steel ribbon comprising each of said U-shaped pieces being solidly held together with intervening thin layers of a thermoset bonding material adhering to the layers of magnetic sheet steel ribbon, and a resilient strap of steel applied under tension about the two U-shaped pieces to maintain constant pressure between their adjoining butt Joint surfaces.

5. In an electrical apparatus, a magnetic core structure comprising a core loop formed from a ribbon of magnetic sheet steel wound flatwise layer upon layer about an eiongated window and having two opposite sides longer than the remaining two sides, the two longer sides of the core loop being formed with a slight bow outward from each other along the central portions thereof, the departure of each longer side of the core loop from a straight line between its ends being in the order of two percent of the length of the side with the greatest departure of the side from a straight line being at the midpoint of the side, the two longer sides being cut across midway between their ends forming two substantially U-shaped core loop sections having leg portions of equal length terminating in adjoining butt Joint surfaces, the adloining surfaces being machine worked and etched to form close fitting smooth butt joint surfaces, the layers of magnetic sheet steel ribbon comprising each of said U-shaped sections being solidly held together with intervening thin layers of a thermoset bondingV material adhering to the layers of magnetic sheet steel ribbon, and a resilient band of metal applied under tension about the two U-shaped core sections to maintain constant pressure between their adjoining butt Joint surfaces.

6. In an electrical apparatus, a magnetic core structure comprising a core loop formed from a ribbon of magnetic sheet steel wound flatwise layer upon layer about a substantially rectangular window presenting a side surface of the ribbon steel along the surface of the core outlining the window, two opposite sides of the core loop being formed with a slight curvature outwardly from each other, the departure of these sides from a straight line between their ends being in the order of two per cent of the length of the side with the greatest departure of the side from a straight line being at the midpoint of the side, the curved sides being cut midway between their ends forming two separate U-shaped pieces having abutting joints between the respective 'ends of the two pieces, the layers of magnetic sheet steel ribbon comprising each of said U-shaped pieces being solidly held together with intervening thin layers of a bonding material adhering to the layers of the magnetic sheet steel ribbon, and a resilient strap of steel applied under tension about the two U-shaped pieces to maintain constant pressure between their adjoining butt joint surfaces.

'i 'JAMES G. FORD.

CLIFFORD C. HORSTMAN. JOHN H. BRAMBLE.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PA'I'ENTS Number Name Date 714,891 Gill Dec. 2, 1902 2,142.066 Eppelsheimer Dec. 20, 1938 2,260,398 Otte Oct. 28, 1941 2,374,449 Mulcahy Apr. 24, 1945 2,400,994 Horstman et al. May 28, 1948 

